Notes on Energy | Grade 8 > Science > Work, Energy and Power | KULLABS.COM

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The capacity of a body to do work is called the energy of the body. The SI unit of energy is joule (J). Energy can exist in various forms. Different forms of energy are:

1. Mechanical energy: Energy by the virtue of which a body can do some mechanical work directly, is called mechanical energy. The common forms of the mechanical energy are:

• Kinetic energy:The energy possessed by a body due to its motion is called kinetic energy. Kinetic energy (K. E) of a body having mass m, and velocity v, is equal to equation That is, K. E = mv2

• Potential energy:The energy possessed by a body due to its position or configuration is called potential energy. Potential energy (P.E) of a body is P.E = m x g x h

Where, m is a mass of the body, g is the acceleration due to gravity and h is height.

2. Electrical energy: A form of energy created by the movement of electrons is called electrical energy. I.e. hydropower, generator, etc.

3. Light energy: Light is also form of energy that makes things visible. We use light in the different process like lightening the houses, running industries, etc. Plants use energy to make food.

4. Heat energy: Heat energy is obtained by burning combustible substances like coal, gas, kerosene, firewood, etc. We use heat energy to keep warm and to cook food.

5. Sound energy: Sound energy is also a form of energy which is produced by vibrations. We hear the sound when the vibrations travel through the air around it to vibrate.

6. Chemical energy: The energy stored in the matter such as the energy stored in food and fuels, is chemical energy.

7. Nuclear energy: The neutrons and protons exert an attractive force on each other when present in the nucleus. The energy associated with them is called nuclear energy.

8. Magnetic energy: The energy can also be produced by a magnet. A magnet is the source of energy. Electricity can be produced with the help of the magnet. It is more powerful and also can be used to lift a heavy weight.

#### Transformation of energy

Conversion of energy from one form to another form is called the transformation of energy.

When a candle burns, chemical energychangesinto light and heat energy. When a fan is switched on, it converts an electrical energy into mechanical energy. Similarly, when a lamp glows, it changes electrical energy into light and heat energy. These examples show that one type of energy can be changed into another type.

Examples:

1. A steam engine changes heat energy into mechanical energy.
2. Solar car converts solar energy into mechanical energy.
3. A microphone converts sound energy into electrical energy.

#### Conservation of energy

Energy can neither be created nor be destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.

• The capacity of a body to do work is called the energy of the body. The SI unit of energy is joule (J). Energy can exist in various forms.
• Conversion of energy from one form to another form is called transformation of energy.
• This is called the principle of conservation of energy.
.

### Questions and Answers

#### Click on the questions below to reveal the answers

 s.n magnetic energy chemical energy 1 It is produce from magnet. It is due to a chemical reaction. 2 Electricity can be produced with the help of magnet. This energy is released in the form of electrical energy. 3 It is used in radio, telephone, and telegraph etc. Store in matter such as the energy store in food and fuel.

The energy possessed by a body due to its position or configuration is called potential energy. For example, a raised stone has potential energy and can do the work and a stretched rubber band has potential energy because of their stretched condition.

A flying bird possesses both potential and kinetic energies as it is at a certain height above the ground and moving with the certain velocity.

A man is standing at the rest on a moving car does not possess kinetic energy.

Chemical energy is form of energy that the food contains.

Kinetic energy depend on following factors:

• The kinetic energy of a body is directly proportional to the mass of the moving body.
• The kinetic energy of a body is direct proportional to square of velocity of body.

Conversion of energy from one form to another form is called the transformation of energy. Examples of conservation of energy are:

1. A steam engine changes heat energy into mechanical energy.
2. Solar car converts solar energy into mechanical energy.
3. A microphone converts sound energy into electrical energy.

Energy can neither be created nor destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.

\begin{align*} \text{mass}\:(m) &= 65\: kg \\ \text {velocity} \: (v) = 10 km/hr = \frac {10\times 1000 m}{1 \times 60\: \text {min}\times 60\: \text {seconds}}\\ &= 0.2777\: km/hr \\ \text {kinetic energy} (K.E) &= ?\\ \text {We know} \\ K.E. &= \frac {1}{2}mv^2 \\ &= \frac 12 \times 65 \times (0.277)^2 \\&= 2.49 \: \text {joules} \\ \end{align*}

\begin{align*} \text {Given,} \\ \text {mass} \:(m) &= 10 kg \\ \text {height from the ground} \: (h) &= 10\: m \\ \text {acceleration due to gravity} \:(g) &= 9.8 ms^2 \\ \text {potential energy}\: (P.E.) &= ? \\ P.E. &= m\times g\times h \\ &= 10 \times 9.8 \times \: 10 \\ &= 980\: \text {joule} \\ \end{align*}

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• ### One joule is the amount of work done when ______.

one newton force does not covers any distance

one newton force covers a distance of 1 m

one newton force covers distance of 2 m

none of these

• ### Energy is defined as ______.

the capacity to do work

the rate of doing work

the amount of work

the average rate of work

• ### Which statement is true for electrical energy?

It is a form of energy produced by vibrations

It is the energy associated with neutrons and protons

It is the energy possessed by a body due to its motion

It is a form of energy created by the movement of electrons

• ### A rubber band zinged from your finger has ______.

heat energy

kinetic nergy

chemical energy

potential energy

• ### Which of the following are the types of energy?

heat energy

electrical energy

all the answers are correct

mechanical energy

• ### Kinetic and Potential energies are common forms of ______.

heat energy

mechanical energy

nuclear energy

electrical energy

• ### The energy possessed by the moving object is _______.

electrical energy

mechanical energy

kinetic energy

heat energy

• ### Kinetic energy of a body depends on ______.

none of the answers are right

acceleration due to gravity

mass of the body

height raised from the ground

• ### If the velocity of the body is doubled, then the kinetic energy of the body ______.

increases twice the original

increases thrice the original

does not increase at all

increases four times the original

• ### Which one of the following  is the use of kinetic energy?

all of the answers are right.

use of running water to run the water mills.

penetration of the bullet into the target.

use of hammer to push the nail into the wood.

• ### Energy possessed by a body by the virtue of its position or configuration is called ______.

electrical energy

potential energy

heat energy

mechanical energy

• ### Which of the following examples has stored potential energy within it?

a coiled spring

a stretched rubber band

all the answers are correct

a brick raised to a certain height

• ### As we carry a brick upwards the potential energy of that brick increases because _______.

the distance from the ground increases

the acceleration due to gravity increases

the mass of the brick increases

all the answers are correct

• ### A 5 kg rock and a 10 kg rock are dropped from a height of 10 m. Which of the following statements describes what happens to the potential energy of the rocks?

Neither rock loses potential energy because no work is done on the rocks.

The 10 kg rock loses more potential energy than the 5 kg rock because the 10 kg rock falls faster.

The 10 kg rock loses more potential energy than the 5 kg rock because the 10 kg rock has more mass

Both rocks lose the same amount of potential energy because they fall from the same height.

## ASK ANY QUESTION ON Energy

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Forum Time Replies Report
##### What is nuclear energy

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##### Aayush

What is alternative energy?

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##### Sanjeet

An electric bulb is marked with 60W.What does it mean?

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##### sanju

what is meant by 100w marked on a bulb?

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